Recent metal-machining tools, sliding members, etc. are produced by forming hard coatings having high hardness and heat resistance on members of high-speed steel or cemented carbide. Coating technologies include chemical vapor deposition (CVD) for forming hard coatings with residual tensile stress, and physical vapor deposition (PVD) for forming hard coatings with residual compression stress. CVD-coated members are mainly used for turning, while PVD-coated members are mainly used for milling.
Both coating technologies have advantages and disadvantages. For instance, CVD can form coatings as thick as 10 μm or more, which have excellent adhesion to have improved wear resistance, but are poor in chipping resistance because of residual tensile stress. Further, because CVD temperatures are as high as about 1000° C., member materials suitable for CVD are restricted.
PVD coatings have excellent chipping resistance because of residual compression stress. Because of lower coating temperatures than in CVD, member materials suitable for PVD are not restricted. However, because of residual compression stress increasing as the thickness of coatings, PVD coatings of 3 μm or more in thickness are generally poor in adhesion than the CVD coatings. Accordingly, hard PVD coatings should be thinner than hard CVD coatings to secure adhesion. CVD is thus used rather than PVD, for instance, for turning tools requiring wear resistance.
JP 2003-136302 A discloses a hard PVD coating of TiAlN having an average thickness of 2-15 μm, which has high orientation in a (200) plane, and an X-ray diffraction peak whose half-value (2θ) is 0.6° or less. JP 2003-145313 A discloses a hard PVD coating of TiAlSiN having an average thickness of 2-10 μm, which has high orientation in a (200) plane, and an X-ray diffraction peak whose half-value (2θ) is 0.5° or less. JP 2003-136302 A and JP 2003-145313 A describe that cemented carbide tools having highly-oriented, hard PVD coatings of TiAlN or TiAlSiN exhibit excellent wear resistance in high-speed cutting of steel, soft steel, etc. causing high heat generation. However, thick coatings have large strain and thus large residual compression stress, resulting in deteriorated adhesion and chipping resistance.
JP 2003-71611 A discloses a hard coating for a cutting tool, which has a composition of (Ti1-a-b-c-dAlaCrbSicBd)(C1-eNe), wherein a, b, c, d and e are the atomic ratios of Al, Cr, Si, B and N, meeting 0.5≦a≦0.8, 0.06≦b, 0≦c≦0.1, 0≦d≦0.1, 0.01≦c+d≦0.1, a+b+c+d<1, and 0.5≦e≦1, the X-ray diffraction intensities I(111), I(200) and I(220) of a (111) plane, a (200) plane and a (220) plane in the hard coating meeting I(220)≦I(111), I(220)≦I(200), and I(200)/I(111)≧0.3. However, the hard coating in Example of JP 2003-71611 A is as thin as 3 μm, poor in wear resistance. Although a high-hardness coating is formed by as high bias voltage as 100 V or more, the residual compression stress of the coating increases as it becomes as thick as, for instance, 5 μM or more, resulting in decrease in adhesion and chipping resistance.
JP 9-300106 A discloses a cemented carbide insert having a hard coating made of a composite nitride, carbonitride or carbide of Ti and Al, in which a ratio Ib(220)/Ia(111) of the intensity Ib(220) of a (220) plane to the intensity Ia(111) of a(111) plane in an X-ray diffraction is in a range of 1.0<Ib/Ia≦5.0. However, the hard coating described in JP 9-300106 A has poor crystal grain boundary strength (adhesion) at a thickness of, for instance, about 5 μm, though stress can be lowered by controlling the peak intensity of a (220) plane if the coating is as thin as about 3 μm. Defects existing in the crystal grain boundaries increase as the hard coating becomes thicker, thereby extremely deteriorating chipping resistance.
JP 2001-277006 A discloses the inclusion of pluralities of TiN layers in, for instance, a TiAlN layer to reduce residual stress. However, this introduces many lattice defects into the hard coating, particularly defects between the TiN layers and the TiAlN layers. Accordingly, this hard coating has poor resistance to shearing impact and thus poor chipping resistance because the lattice defects act as starting points of breakage when made thicker, despite improved abrasive wear resistance.
JP 8-209334 A and JP 7-188901 A disclose the reduction of residual compression stress in PVD coatings having a thickness of 3-4 μm. The formed hard coatings have high adhesion because of reduced residual compression stress, but are poorer in wear resistance than the CVD coatings because of smaller thickness.
JP 2007-56323 A discloses a hard coating having excellent adhesion and lubrication with a controlled ratio of metal elements to gas elements both forming the hard coating. However, it is difficult to form a uniform hard coating because it has a multi-layer structure in which component ratios change in each layer.
JP 7-157862 A discloses a hard PVD coating having recesses as deep as 0.2-2 μm provided by removing droplets projecting from the surface by cutting or grinding, describing that the smoothening of the hard coating surface improves chipping resistance and wear resistance. It is also described that the recesses on the surface can receive a lubricant. In recent high-performance machining, however, such recesses provide the surrounding coating portions with reduced mechanical strength. The droplets remain in the hard coating as defects even if their portions projecting from the surface are removed. Accordingly, when they are exposed on the surface after machining, the hard coating has poor impact resistance and heat resistance.
JP 2005-335040 A discloses a hard PVD coating having a predetermined surface roughness, which has the minimum thickness t near a cutting edge portion and the maximum thickness T on a face or flank meeting 0≦t/T≦0.8. It describes that surface smoothing provides the hard coating with improved chipping resistance and wear resistance. It further describes that the hard coating surface can be smoothed by various grinding methods such as brushing, blasting, barrel finishing, etc. As described in JP 2005-335040 A, however, the smoothing of the hard coating surface exposes droplets existing in the coating from the surface. Because the exposed droplets are softer than the surrounding hard coating phase, uneven wear occurs locally, resulting in recesses on the surface, which deteriorate seizure resistance. Also, the recessed surface has poor heat resistance, vulnerable to oxidation wear.
JP 2000-34561 A and JP 2003-193219 A disclose methods for reducing droplets in a hard coating. However, they are improvement in an apparatus, failing to eliminate droplets completely. In addition, no consideration is made at all on droplets contained in the hard coating. The droplets contained in the hard coating act as defects, directly deteriorating the properties of the hard coating.
As described above, conventional technologies fail to provide a thick hard PVD coating with high hardness and heat resistance as well as improved chipping resistance and wear resistance, without deteriorating adhesion to a substrate and mechanical strength to impact from outside.